Crosslinked vinylamine polymer in enhanced oil recovery

ABSTRACT

The production of oil or gas from a subterranean formation is enhanced by injecting into the formation, such as in acidized fracturing, an aqueous fluid containing as a viscosifier a crosslinked vinylamine polymer. Acidizing fluids are provided containing HCl and a viscosifying amount of a gel of the crosslinked polymer, preferably poly(vinylamine) crosslinked with a diisocyanate, a polyepoxide or epichlorohydrin.

FIELD OF INVENTION

This invention relates to a method of enhancing the production of oil orgas using a viscosifying agent based upon crosslinked vinylaminepolymer. In another aspect it relates to an acidized fracturing fluidfor use in enhanced oil or gas recovery containing a crosslinkedvinylamine polymer.

BACKGROUND OF THE INVENTION

Polymers are used in a wide variety of ways to enhance the production ofoil or gas from underground formations. Usually the function of thepolymer is to control the viscosity of the aqueous fluids which areinjected into the formation. For example, in water flooding theefficiency of the water flood is improved by adding a water solublepolymer to the aqueous phase and thereby decreasing the mobilitydifference between the injected water and the oil in place. Polymers arealso used in acidizing and/or fracture acidizing in which acidiccompositions are used to stimulate production of hydrocarbon fromunderground formations by increasing the formation porosity. A watersoluble or water dispersible polymer is incorporated to increase theviscosity of the fluid so that wider fractures can be developed and liveacid can be forced farther into the formations. This increases theproppant carrying capacity of the acid solutions and permits betterfluid loss control.

Generally high molecular weight polymers (those having a molecularweight on average of at least 10⁶) or polymers with various gelling orcrosslinking agents are used for this purpose. Most commerciallyavailable polymeric viscosifiers, however, are degraded by the hostilereservoir environment including high temperatures, acidity and extremeshear conditions, as well as by the electrolytes which are encounteredin the oil recovery process. For example, hydrolyzed polyacrylamidesfail in sea water solution at elevated temperatures due to precipitationof the polymer in the presence of calcium ions in the sea water. Xanthanpolymers are insensitive to calcium ions but these polymers degrade athigh temperatures and lose their viscosifying efficiency.

U.S. Pat. No. 4,579,667, Echt, et al. (1986) discloses gelled aqueouscompositions containing an anionic partially hydrolyzed acrylamide and awater-soluble cationic polyamide-epihalohydrin resin useful infracturing oil and gas formations in enhanced oil recovery. Theinvention is said to eliminate the need to use polyvalent metal saltsand complexes in order to convert a water-soluble polymer to acrosslinked gel. Gels can be used to suspend propping agents, to reducewater loss by serving as a plugging agent, and as a "pusher" insurfactant flooding.

U.S. Pat. No. 4,690,219, Burns, et al. (1987) discloses acidizing wellstimulation for oil production using an acid solution thickened with acopolymer of an N-vinyl lactam, such as N-vinyl-2-pyrrolidone, and an α,β-unsaturated amide, such as acrylamide. The thickening polymer operatesin the absence of a crosslinking agent, and is said to function inhostile formation environments including temperatures above 170° F.Aldehydes are disclosed, however, as suitable crosslinking agents toform a gel.

It is known that crosslinked polyvinylamines can be prepared in variousways. For example, Japanese Patent Publication No. J61051007-A (1986)discloses making a crosslinked poly(vinylamine) by copolymerizingN-vinylformamide with a copolymerizable crosslinkable monomer and thenhydrolyzing the amide groups to amine. Japanese Patent Publication No.J61051006-A (1986) discloses, on the other hand, suspending apolyvinylamine in a dispersion medium and then crosslinking the polymerby reacting it with a dialdehyde or epichlorohydrin.

The preparation of polyvinylamines by hydrolysis of N-vinylformamidepolymers is likewise well known. U.S. Pat. No. 4,623,699, Brunnmueller,et al. (1986) discloses making poly(vinylamines) by eliminating formylgroups from a polymer formed from N-vinylformamide using gaseoushydrogen chloride in the presence of not over 5 wt. % water, based onthe polymer.

This application is related to U.S. Pat. No. 4,843,118 of Lai andVijayendran which issued June 27, 1989, based on application Ser. No.64,962 filed June 19, 1987. This patent discloses an acidized fracturingfluid containing poly(vinylamine) obtained by at least 50% hydrolysis ofa poly(vinylamide) having a molecular weight of at least 10⁶. Backgroundpatents are referenced as disclosing methods for makingpoly(vinylamines) having lower molecular weights, normally involvingsolution polymerization. Crosslinking agents are optional and includeorganic titanate complexes, epichlorohydrin, hexamethylene diisocyanate,glyoxal, butanediol diacrylate, terephthaldehyde and glutaraldehyde. Thedisclosure of this patent, which is incorporated herein by reference,was filed as a continuation-in-part of Ser. No. 914,046, filed Oct. 1,1986. This parent application and a related European Patent ApplicationNo. 0,264,649, published April 24, 1988, disclose the preparation ofpoly(vinylamines) having at least 10⁶ average molecular weight by usingeither acid or base hydrolysis of poly(N-vinylformamide). Thepolymerization technique used is inverse emulsion polymerization. Amongthe utilities disclosed for such high molecular weight amines are usesin drilling mud compositions, cements for drilling holes, completionfluids, acidized fracturing fluids and general use in enhanced oilrecovery. The parent application and the European patent application donot, however contain the disclosure found in the '118 patent concerningthe use of crosslinking agents which can be incorporated into acidizedfracturing fluids for well stimulation when such fluids contain highmolecular weight poly(vinylamine). Application Ser. No. 914,046 isabandoned but the corresponding disclosure was filed in foreigncountries and issued as Taiwan Patent No. 31922 on Aug. 10, 1989.

It is highly desirable to develop polymer gels which are stable and canbe used as plugging agents in enhanced oil recovery. Such polymer gelsare placed in high permeability zones in a formation so thatsubsequently injected treatment fluid is forced into the lowpermeability zones thereby giving better sweep efficiency by increasingthe contact of the flooding fluid with the oil bearing reservoir sands.

BRIEF SUMMARY OF THE INVENTION

We have found that vinylamine polymers over a broad range of molecularweights can be crosslinked and used effectively because of theirdemonstrated stability under harsh environmental conditions in enhancedoil recovery. These polymers demonstrate good stability at hightemperatures under acid conditions and high electrolyte concentrationsand are particularly suitable for use in acid or matrix fracturing ofoil or gas bearing formations. Accordingly our invention provides amethod of enhancing the production of oil or gas from a subterraneanformation by injecting into the formation a fluid which contains as aviscosifier a vinylamine polymer which has been crosslinked with eithera multi-functional organic compound or an inorganic compound containingmulti-valent anions such as titanates, zirconates, phosphates,silicates, etc. or an inorganic cation such as Cu⁺², Fe⁺³, Fe⁺², Zn⁺²,etc. which is capable of complexing with said vinylamine polymer. Thecrosslinked vinylamine polymer has greatly increased viscosity andpreferably is in the form of a gel. Crosslinking agents which areparticularly advantageous are diepoxides and diisocyanates.

The invention also provides an acidized fracturing fluid which issuitable for use in enhanced oil or gas recovery and comprises water, anacidizing amount of hydrochloric acid and a viscosifying amount of thecrosslinked vinylamine polymer described above. By using thecrosslinking agent to form high viscosity polymers or, preferably,polymer gels, the original vinylamine polymer can have a molecularweight substantially less than 10⁶ as required in U.S. Pat. No.4,843,118.

DETAILED DESCRIPTION OF THE INVENTION

Poly(vinylamines) can be prepared by a variety of different methods asreferenced above in the background of the invention. The molecularweight of the poly(vinylamines) used in this invention can range from aslow as 100,000 to 10 million or more, but is preferably in the range ofabout 0.5 to 1 million. The preferred method of making thesepoly(vinylamines) is by hydrolysis of a poly(N-vinylformamide). It iswithin the scope of this invention, however, to use copolymers ofvinylacetate and N-vinylformamide which on hydrolysis, either in asingle or two-phase procedure, produce a copolymer containing poly(vinylalcohol) linkages and poly(vinylamine) linkages. Polymers which contain25% or more of their vinyl linkages as a poly(vinylamine) can becrosslinked and used to practice this invention as described and it isintended that the term, vinylamine polymer, encompass such copolymerscontaining the vinylamine monomer linkages with or without copolymerizedmonomer containing the vinyl alcohol units. Also within the scope ofthis invention are the use of such polymers containing vinylamine unitsin which the parent polymer is not completely hydrolyzed and, forexample, contains up to 15 mol % but preferably not over 4 mol % of thepolymerized vinyl units containing the original amide or formyl groups.If a polymer is formed with vinylacetate as a comonomer the polymercould also contain unhydrolyzed acetate groups in approximately the samelimited amounts.

The crosslinking agents which can be used are either multi-functionalorganic compounds such as dialdehydes, polyepoxides, di- ortriacrylates, di- or triisocyanates or dihalides, or inorganic compoundscontaining multi-valent anions or inorganic cations which are capable ofcomplexing with poly(vinylamine). Among these agents it is preferred toform polymer gels with either the diepoxides or diisocyanates as thegels formed with these crosslinking agents show higher stability thanthose formed with the dialdehydes, dihalides, or diacrylates. Thisstability is exhibited in acid solution at high temperatures such aswould be encountered by a well injection fluid at bottom-hole conditionsof an oil or gas producing well.

The viscosity of the polymer solution which is used to form theinjection fluids depends on the molecular weight, the concentration ofthe poly(vinylamine), the degree of crosslinking, pH and electrolyteconcentrations. Poly(vinylamines) with very high molecular weightsexceeding 10⁶ at high concentrations can give a desired viscosifyingeffect in the absence of crosslinkers as described in our U.S. Pat. No.4,843,118. We have found, however, that in order to prepare thevinylamine polymer gels which are found especially desirable, suitableamounts of crosslinker are required. The amounts of crosslinker to beused ranges from 0.01 to about 30 mol % relative to homopolymer aminegroups, and from 0.01 to about 50 mole % of the amine content ofvinylamine copolymers. The thermal stability of the vinylamine polymergels depends upon the strength of these crosslinking bonds. We havefound quite surprisingly that medium molecular weight poly(vinylamines),for example those having a molecular weight above 100,000 but less than1 million, can be converted to polymer gels by crosslinking, andespecially by crosslinking with diisocyanates or diepoxide compounds,and that such gels are quite stable in acid solution at hightemperatures.

The concentration of the polymer gel or crosslinked polymer in thewell-treating fluid depends on the molecular weight of the polymer andthe crosslink density, but it is generally in the range of about 0.01 to10 wt % of the well-treating fluid and preferably in the range of 0.1 toabout 5 wt %.

In order to describe our invention further the following examples arepresented which are meant to be illustrative only and not to limitunduly the scope of our invention.

EXAMPLE 1 Preparation of poly(vinylamine)

N-vinylformamide (NVF) was polymerized to form an aqueous solution ofpoly-NVF. To a 22 L three-necked round-bottomed flask were charged 900 gof NVF, 8.1 kg of H₂ O and 3.15 g of Vazo 52 (2,2'-azobis(2,4-dimethylpentanenitrile) from E. I. DuPont de Nemours & Co.). The solution pH wasadjusted to 8.5 with NH₄ OH. The polymerization was carried out at 60°C. for 4 hours under a nitrogen atmosphere.

The aqueous poly-NVF solution thus obtained was mixed with an equimolaramount of 50% aqueous NaOH solution. The resulting mixture was heatedfor 8 hours at 80° C. under a nitrogen atmosphere. To the reactionmixture was added concentrated hydrochloric acid (˜3 L) until thepolymer precipitated. The acid solution was decanted. The precipitatedpolymer was redissolved in water and reprecipitated with methanol (˜20gallons for 1 kg of polymer). The polyvinylamine had a weight averagemolecular weight of 600,000.

EXAMPLE 2 Crosslinking poly(vinylamine)

Poly(vinylamine . HCl) prepared in Example 1 was dissolved in water togive a 2.5 weight % solution. Brookfield viscosity of the stock solutionwas 330 cps at pH of 6.5. To separate portions of this solution wasadded 5 mole % (unless otherwise indicated) based upon the polymer ofnine different crosslinking agents and NaOH was added in amountsnecessary to obtain the desired solution pH. The resulting solutionswere stirred at room temperature for 1 hour after which solutionviscosity was measured by using a Brookfield Viscometer. Results withvarious crosslinkers are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Crosslinker   pH    Temp. (°C.)                                                                        Viscosity (cps)                               ______________________________________                                        Glutaraldehyde                                                                              3     25          gel (immediately)                             Glutaraldehyde                                                                              4     25          gel (immediately)                             Glutaraldehyde                                                                              5     25          gel (immediately)                             Glutaraldehyde                                                                              6     25          gel (immediately)                             Glutaraldehyde                                                                              7     25          gel (immediately)                             Glutaraldehyde                                                                              8     25          gel (immediately)                             Glutaraldehyde                                                                              9     25          gel (immediately)                             Glutaraldehyde                                                                              10    25          gel (immediately)                             Glutaraldehyde                                                                              11    25          gel (immediately)                             Butanediol Diacrylate                                                                       3     25          476                                           Butanediol Diacrylate                                                                       4     25          398                                           Butanediol Diacrylate                                                                       5     25          484                                           Butanediol Diacrylate                                                                       6     25          354                                           Butanediol Diacrylate                                                                       7     25          396                                           Butanediol Diacrylate                                                                       8     25          gel (60 min.)                                 Butanediol Diacrylate                                                                       9     25          gel (20 min.)                                 Butanediol Diacrylate                                                                       10    25          gel (10 min.)                                 Butanediol Diacrylate                                                                       11    25          gel (10 min.)                                 Butanediol Diacrylate                                                                       6     90          gel (30 min.)                                 Butanediol Diacrylate                                                                       7     90          gel (10 min.)                                 Diisocyanatohexane                                                                          3     25          540.sup.(1)                                   Diisocyanatohexane                                                                          4     25          678.sup.(1)                                   Diisocyanatohexane                                                                          5     25          gel (30 min.)                                 Diisocyanatohexane                                                                          6     25          900                                           Diisocyanatohexane                                                                          7     25          460                                           Diisocyanatohexane                                                                          8     25          178                                           Diisocyanatohexane                                                                          9     25          116                                           Diisocyanatohexane                                                                          10    25           68                                           Diisocyanatohexane                                                                          11    25           62                                           Epichlorohydrin                                                                             3     25          410                                           Epichlorohydrin                                                                             4     25          324                                           Epichlorohydrin                                                                             5     25          346                                           Epichlorohydrin                                                                             6     25          306                                           Epichlorohydrin                                                                             7     25          194                                           Epichlorohydrin                                                                             8     25          150                                           Epichlorohydrin                                                                             9     25          106                                           Epichlorohydrin                                                                             10    25          gel (105 min.)                                Epichlorohydrin                                                                             11    25          gel (30 min.)                                 Epichlorohydrin                                                                             9     90          gel (10 min.)                                 Hylite HF.sup.(2)                                                                           3     25          858                                           Hylite HF     4     25          646                                           Hylite HF     5     25          446                                           Hylite HF     6     25          330                                           Hylite HF     7     25          256                                           Hylite HF     8     25          182                                           Hylite HF     9     25          140                                           Hylite HF     10    25          124                                           Hylite HF     11    25          822                                           Hylite HF     7     90          1048.sup.(3)                                  Hylite HF     8     90          gel (60 min.)                                 Cymel 303.sup.(4)                                                                           3     25          446                                           Cymel 303     4     25          724                                           Cymel 303     5     25          426                                           Cymel 303     6     25          310                                           Cymel 303     7     25          248                                           Cymel 303     8     25          190                                           Cymel 303     9     25          124                                           Cymel 303     10    25           66                                           Cymel 303     11    25           58                                           Dimethyl adipate                                                                            3     25          578                                           Dimethyl adipate                                                                            4     25          484                                           Dimethyl adipate                                                                            5     25          308                                           Dimethyl adipate                                                                            6     25          236                                           Dimethyl adipate                                                                            7     25          218                                           Dimethyl adipate                                                                            8     25          164                                           Dimethyl adipate                                                                            6     75          796                                           Tyzor AA.sup.(5)                                                                            3     25          738                                           Tyzor AA      4     25          620                                           Tyzor AA      5     25          gel (20 min.)                                 Tyzor AA      6     25          gel (20 min.)                                 Tyzor AA      7     25          gel (20 min.)                                 Glyoxal.sup.(6)                                                                             3     25          352                                           Glyoxal       4     25          450                                           Glyoxal       5     25          468                                           Glyoxal       6     25          gel (immediately)                             Glyoxal       7     25          gel (immediately)                             Glyoxal       8     25          gel (immediately)                             Glyoxal       9     25          gel (immediately)                             Glyoxal       10    25          gel (immediately)                             Glyoxal       11    25          gel (immediately)                             Glyoxal       3     90          gel (60 min.)                                 Glyoxal       4     90          gel (60 min.)                                 ______________________________________                                         .sup.(1) gel overnight at 25° C.                                       .sup.(2) Hylite HF is a partially alcohol protected                           N,N'dihydroxymethyl4,5-dihydroxyethyleneurea.                                 .sup.(3) gel overnight but still flows.                                       .sup.(4) Cymel 303 is a hexamethoxymethyl melamine resin from American        Cyanamid.                                                                     .sup.(5) Tyzor AA is organic titanate from DuPont and was added in 2 mole     % proportions instead of 5 mole %.                                            .sup.(6) Added in 10 mole % proportions instead of 5 mole %.             

EXAMPLE 3

A procedure as described in Example 2 was followed to show the effect ofvarious levels of the crosslinking agent, glutaraldehyde, which producedgels immediately at a 5 mole percent level (see Table 1).

A 2.5 wt. % solution of the poly(vinylamine) of Example 1 was used. Thetemperature was 25° C. and the pH was 6. Results are given in Table 2.

                  TABLE 2                                                         ______________________________________                                        GLUTARALDEHYDE (Mole %)                                                                            VISCOSITY (cps)                                          ______________________________________                                        0                      332                                                    0.5                  1,380                                                    1                    11,500                                                   >2%                  GEL                                                      ______________________________________                                    

These data show that viscosity of the poly(vinylamine) solution and gelproduction can be easily controlled by the proportion of crosslinkeradded.

EXAMPLE 4

A procedure as described in Example 2 was followed to show the effect ofpolymer concentration on polymer solution viscosity. Glutaraldehyde wasadded at a 5 mole percent level as in Example 2, using a temperature of25° C. and a pH of 6. As shown in Table 1, with a polymer concentrationof 2.5 weight percent, this level of crosslinker formed a gelimmediately. The polymer used in this example was also poly(vinylamine)having a molecular weight before crosslinking of 600,000. Results aregiven in Table 3.

                  TABLE 3                                                         ______________________________________                                        Polymer Concentration                                                         (weight %)       VISCOSITY (cps)                                              ______________________________________                                        >1.5%            GEL                                                           0.5%            206                                                          ______________________________________                                    

The above data show that solution viscosity can also be controlled byadjusting the level of the poly(vinylamine). The viscosity of the 0.5 wt% solution before crosslinking the polymer was 30 cps.

EXAMPLE 5

The stability of poly(vinylamine) gels made with various crosslinkerswas tested by immersing the polymer gels in 1.5N HCl acid solution at90° C. The polymer gels were made by the procedures described inExamples 1 and 2. The results summarized in Table 4 show that thepoly(vinylamines) crosslinked with diisocyanatohexane and diepoxidesformed more stable gels than those formed with the other crosslinkingagents tested.

                  TABLE 4                                                         ______________________________________                                        STABILITY OF POLY(VINYLAMINE) GEL                                             CROSSLINKER            GEL                                                    ______________________________________                                        Glyoxal                BREAK                                                  Butanediol diacrylate  BREAK                                                  Epichlorohydrin        BREAK                                                  Diisocyanatohexane     STABLE                                                 Tyzor AA               BREAK                                                  Diepoxides             STABLE                                                 ______________________________________                                    

EXAMPLE 6

Crosslinked polymer gels were prepared from a copolymer of vinyl alcoholand vinylamine prepared by copolymerizing vinyl acetate andN-vinylformamide followed by hydrolysis of the acetate group to alcoholgroups and hydrolysis of the amide groups to amine. The copolymercontained 30 mole percent vinylamine units and had a weight averagemolecular weight of 36,000. To an aqueous solution of 2.5 weight percentof the copolymer (PVOH/30% PVAm) was added various amounts of butanedioldiacrylate (BD) at 90° C. and a pH of 9. The time required for a gel toform in each case is reported in Table 5.

                  TABLE 5                                                         ______________________________________                                        Mole % BD                                                                     Based on Polymer                                                                              Gel Time (min)                                                ______________________________________                                        20%             20                                                            30%             15                                                            40%             10                                                            ______________________________________                                    

The above data show that even relatively low molecular weight polymerscontaining only 30 mole percent amine units can be converted to gels bycrosslinking. Such polymers in their gelled state are useful in ourinvention.

Other aspects and embodiments of our invention will be apparent to thoseskilled in the art without departing from the spirit or scope of ourinvention.

We claim:
 1. An acidified fracturing fluid suitable for use in enhancingoil or gas recovery from an oil or gas bearing formation comprisingwater, an acidifying amount of hydrochloric acid and a viscosifyingamount of a vinylamine polymer crosslinked with a multi-functionalorganic compound or an inorganic compound containing multi-valent anionsor an inorganic compound capable of complexing with saidpoly(vinylamine).
 2. The composition of claim 1 wherein said polymer ispoly(vinylamine).
 3. The composition of claim 2 wherein saidpoly(vinylamine) prior to crosslinking has an average molecular weightless than 10⁶.
 4. The composition of claim 2 wherein said crosslinkedpoly(vinylamine) forms a gel.
 5. The composition of claim 4 wherein saidcompound used for crosslinking is a polyepoxide or epichlorohydrin. 6.The composition of claim 4 wherein said compound used for crosslinkingis a diisocyanate.
 7. The composition of claim 1 wherein said polymer isa copolymer of vinyl alcohol and vinyl amine.
 8. The composition ofclaim 1 wherein said multi-valent anions are selected from the groupconsisting of titanates, zirconates, phosphates and silicates.